A method for inkjet recording has advantages in that it facilitates full-color printing because processes in the method for inkjet recording is simpler than processes in other methods for recording and it allows for obtaining high-resolution images even by a simply structured apparatus of the method for inkjet recording.
As an ink used in such a method for inkjet recording, a dye-based ink is typically used in which various water-soluble dyes are dissolved in water or a mixed liquid containing water and an organic solvent, however, dye-based inks have a disadvantage in that they are poor in light resistance, although they are excellent in color tone sharpness. On the other hand, a pigment-based ink in which a carbon black or various organic pigments are dispersed is actively studied because of its excellence in light resistance, as compared to dye-based inks.
However, the pigment-based inks tend to easily cause nozzle clogs as compared to dye-based inks.
The pigment-based ink is prepared by preparing a dispersion in which a coloring material and a dispersant are preliminarily dispersed in an aqueous solvent such as water and alcohols, dispersing the dispersion to a predetermined degree using a media type dispersing device such as sand mill, and then diluting the dispersion to a predetermined concentration.
Typically, in a water-based pigment ink, a surfactant or a water-soluble resin is used for dispersing a hydrophobic pigment, however, the reliability of quality of obtained images with the use of such a water based pigment ink is extremely poor. Then, aiming to improve the image quality, a technique is proposed in which film-formable fine resin particles are added in an ink liquid. However, it is difficult to finely and stably disperse a plurality of components in a liquid for a long period of time. When a large amount of a dispersant such as surfactant is used for stably dispersing these fine particles, air bubbles occur in an ink tank and an ink head, or image quality is degraded. For improving dispersibility of these components, considered are a method of changing the surfaces of a pigment to be hydrophilic, and a method of using a resin containing a hydrophilic group(s), and the like. However, these methods have a problem that when these methods are applied in combination, the dispersion becomes unstable and the storage stability is degraded, although when these methods are applied separately, each resultant dispersion is stable.
To solve the above described problems, proposed are a method of incorporating in an ink a water-insoluble coloring material for obtaining high image density and chargable resin pseudo fine particles having a size smaller than the coloring material (see Patent Literature 1); a method of incorporating in an ink a self-dispersible pigment in a limited absorption amount of DBP oil (see Patent Literature 2); and a method of using a water-based carbon black dispersion liquid with an HLB value of 7 to 18, in which a surface-modified carbon black and a nonionic surfactant having an acetylene skeleton are contained (see Patent Literature 3).
In addition, for the purpose of stabilizing the dispersion of these components in a water-based ink liquid, proposed are a method of dispersing in a water a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group within a molecule (see Patent Literature 4), a method of conforming the polarity of a water-soluble polymer to that of a surfactant or adding a nonionic surfactant in a water-based liquid (see Patent Literature 5), a method of conforming the polarity of a colored ionic polyester resin to that of a hydrophilic group of a colorant in an aqueous recording liquid (see Patent Literature 6), and a method of conforming the dispersion polarity of a pigment to that of a resin fine particle (see Patent Literature 7).
Further, a water-based inkjet ink composition is proposed, which contains a pigment dispersion liquid containing pigment particles in which at least 70% of the particles in the dispersion liquid have diameters less than 0.1 μm and the other particles in the dispersion liquid have a particle size distribution with a diameter of 0.1 μm or less, an aldehyde naphthalenesulfonate dispersant, and/or at least one sulfone solvent (see Patent Literature 8). Patent Literature 9 proposes a recording liquid composed of a water-based medium which contains a pigment, a polymer dispersant and a nonionic surfactant. Furthermore, Patent Literature 10 proposes use of an AB block copolymer or a BAB block copolymer as a dispersant for pigments. Moreover, Patent Literature 11 proposes use of a specific pigment, water-soluble resin and solvent.
Meanwhile, as a method of dispersing a pigment without using a dispersant, proposed are, for example, a method of introducing substituents containing a solubilizing group(s) for water into a carbon black (see Patent Literature 12), a method of polymerizing a water-soluble monomer etc. to the surface of a carbon black (see Patent Literature 13), and a method of subjecting a carbon black to an oxidative treatment (see Patent Literature 14). A method is also proposed which ensures water resistance and ejection stability of an ink through the use of an ink containing a carbon black that has been subjected to an oxidative treatment, and a ternary copolymer composed of acrylic acid, styrene and a-methyl styrene (see Patent Literature 15).
Further, an inkjet recording liquid is proposed in which the volume average particle diameter of dispersion particles is 30 nm to 200 nm (see Patent Literature 16).
As for the above-mentioned inkjet recording liquids, color pigment inks actually allow for obtaining high image densities, however, black pigment inks have not yet been able to provide enough performance to satisfy users. In addition, a technique is proposed in which a bead of a bead size of about 0.05 mm to 1.0 mm is used in bead mill dispersion (see Patent Literature 17). However, this proposal has not been able to provide sufficient dispersion stability.
Further in Patent Literature 18, as a dispersant used is an anionic surfactant, which preferably has a molecular weight in the range of 1,000 to 30,000. However, this proposal does not provide sufficient dispersion stability, involves a type of pigment which is weak in resistance to impact caused by dispersion treatment and thus lacks stability after dispersion treatment, which leads to degradation of ejection stability of an ink.
Thus at present, an inkjet recording ink capable of providing high image density and excellent storage stability thereof and a pigment dispersion liquid therein and preventing the occurrence of clogging at inkjet heads, and its associated technology have not been provided yet.
[Patent. Literature 1] Japanese Patent Application Laid-Open (JP-A) No.2006-8858
[Patent Literature 2] JP-A No.2002-3767
[Patent Literature 3] JP-A No.2006-219584
[Patent Literature 4] JP-A No.05-239392
[Patent Literature 5] JP-A No.08-283633
[Patent Literature 6] JP-A No.2000-63727
[Patent Literature 7] JP-A No.2001-81366
[Patent Literature 8] JP-A No.08-333531
[Patent Literature 9] JP-A No.56-147871
[Patent Literature 10] U.S. Pat. No.5,085,698 [Patent Literature 11] U.S. Pat. No.5,172,133
[Patent Literature 12] U.S. Pat. No.5,571,311
[Patent Literature 13] JP-A No.08-81646
[Patent Literature 14] JP-A No.08-3498
[Patent Literature 15] JP-A No.09-194775
[Patent Literature 16] JP-A No.2000-144028
[Patent Literature 17] JP-A No.2005-281691
[Patent Literature 18] Japanese Patent (JP-B) No. 3625595